1. Field of the Invention
The present invention relates to an image reading apparatus that reads an image on a document, and more particularly to an image reading apparatus that uses a light source configured with an array of point light sources such as LED (Light Emitting Device) to apply light onto a document, and reads an image by receiving reflected light from the document.
2. Description of the Related Art
It is known to provide an image reading apparatus that automatically reads image information of documents has been used as a reading apparatus such as a copier and a facsimile, and a scanner for input to a computer. This type of image reading apparatus uses a light source extended in a fast-scanning direction with respect to a fed document to apply light to it, and receives reflected light reflected from the irradiated document in a reading section to read an image.
This type of image reading apparatus may suffer variances in quantities of light applied from a light source depending on positions in the fast-scanning direction, and secular changes in light quantities of the light source. The variances and changes in light quantities will cause output image signals to vary depending on light quantities of a light source to be outputted when a document on which a half-tone image is formed uniformly is read.
In a conventional image reading apparatus, e.g., a fluorescent lamp has been used as a light source. Recently, however, a LED array configured with an array of plural LEDs is being used to miniaturize an apparatus and reduce power consumption. The fluorescent lamp can be regarded as a line light source and its illumination distribution is substantially constant in the fast-scanning direction. On the other hand, the LED array can be regarded as a light source configured with an array of plural point light sources, and its illumination distribution has a form waving in the fast-scanning direction, with a peak in a location corresponding to main light of each LED. As a result, when a LED array is used as a light source, obtained shading data also has a wavy form corresponding to the illumination distribution.
However, in the case where a LED array is used as a light source, the following problems occur.
For example, when a fed document inclines with respect to a reading face (white reference face) of a white reference member, if image data obtained by reading the inclined document is subjected to shading correction using shading data obtained in advance by reading the white reference member, since a reflection direction of light applied onto the document from a light source differs from that at the time of shading data acquisition, density nonuniformity will remain in the image data posterior to the shading correction in the fast-scanning direction.
Also, in the case where LED arrays are respectively disposed upstream and downstream of a document feeding direction with a reading section sandwiched, and LEDs constituting the each LED are disposed in a staggered fashion, if a fed document inclines, the balance between reflected light entering the reading section based on irradiation light by the upstream LED array and reflected light entering the reading section based on irradiation light by the downstream LED array will collapse. In such a case, if image data obtained by reading the inclined document is subjected to shading correction using shading data obtained in advance by reading the white reference member, density nonuniformity will occur due to a difference between illumination distributions during shading data acquisition and during reading.